France opens probe against Assad regime

French authorities have launched an investigation against the regime of Syrian President Bashar al-Assad for alleged crimes against humanity committed between 2011 and 2013, a judicial source says.

Paris prosecutors opened a preliminary inquiry on September 15, the source said on Wednesday (local time).

The investigation is drawing in particular on evidence provided by a former Syrian military police photographer, who fled the country in 2013, bringing with him about 55,000 graphic photographs of scenes from the brutal conflict.

The announcement comes as the four-year war in Syria takes centre stage at the United Nations General Assembly in New York, where US President Barack Obama and his Russian counterpart, Vladimir Putin, have clashed over how to bring an end to the crisis.

French President Francois Hollande has joined Obama in insisting Assad cannot play a role in the country's future, against opposition from Damascus' allies, Russia and Iran.

"Russia and Iran say they want to be part of a solution," Hollande said.

"So we must work with these countries to explain to them that the route to a solution does not go through Bashar al-Assad."